Description: when mapping a case with lagrangian particles to a smaller mesh some of the particles keep the cell number (in the positions-file) from the old mesh which are a) wrong b) may be outside of the range of the new mesh which causes all kinds of trouble (during decomposePar, trackToFace etc)

Fix: the attached patch. For the particles in question it overwrites the celli_ with the index of the found cell (which was calculated anyway). In order to do this a write-access-method for celli_ had to be introduced